Thank you!
General inspiration was from using SSGI in UE4 and seeing how well it worked there.
The idea to use rough glossy BSDF as a stand in for diffuse, since it has SSR, is not a very novel one. I’d bet that the glossy look of objects while the shaders are compiling has given plenty of ideas.
I spent some time looking if I could add SSR natively on actual diffuse materials without changing the base look, with modifing source code, but since adding a second layer of SSR is far too complex for me, then the way I was trying to do didn’t really seem to have any big advantage of doing similar approach with nodes instead.
I posted most of my tests here: https://blenderartists.org/t/fake-ssgi-in-eevee/.